The DRAGON System

The DRAGON apparatus is what is known as a recoil mass spectrometer. A spectrometer is an instrument that is able to separates a things according to a set of criteria. The simplest example of a spectrometer is a prism. A prism can separate white light into all its components wavelengths: red, orange, yellow, green, blue, indigo, and purple. A mass spectrometer (such as DRAGON) separates not light but particles. Given a mixture of particles, it will separate them according to their mass.

Before DRAGON can do its job, we have to create some particles to separate! This is done in the gas target. We fill the target with either hydrogen or helium (their nuclei being protons and alpha particles respectively), and pass a beam of radioactive particles generated by the ISAC facility through it. In order that the beam may pass unobstructed, an opening is located on either side of the target (the "enter" and "exit", repsectively). This poses some problems, however, because the beamline needs to stay as close as possible to a perfect vacuum. This problem has been solved by "simply" pumping away any gas that might leak out of the gas target as quickly as possible.

The 'recoils' DRAGON was designed to detect are simply the product of the nuclear reaction which is being studied. For example, when studying a beam of 21Na (sodium with 11 protons and 10 neutrons) the recoils we would be separating and then detecting are 22Mg (magnesium with 12 protons and 10 neutrons). The tricky part is separating the recoils from the incoming beam. This is made especially difficult because there are so few recoils compared to beam particles. (roughly, we expect one recoil for every 1x108 of incoming particles). This is where the mass spectrometer comes in.

The mass spectrometer has two components that work together to separate the particles. These are the magnetic dipoles and the electric dipoles. The magnetic dipoles are able to separate the particles by their charge, since particles with different charge are bent by different amounts in a magnetic field. The magnet is set so that it will bend the charge state we are interested in (the one the recoils are most likely to be found in) just the right amount so that the particles with that charge state pass through the charge slits and all the others get trapped in the charge slit box.

The particles leaving the magnetic dipoles have one charge state but a mixture of masses. To separate the remaining particles by mass, we use an electric dipole. The beam particles and the recoil particles both leave the gas target with the same momentum. This is useful because if they both have the same momentum but different masses, they will have different velocities, and hence, different kinetic energies. The electric dipole is able to separate particles based on their kinetic energies. As with the magnetic dipole, we chose the kinetic energy we are interested in (that of the recoils), and apply the right charge to the electric dipole so that the recoils pass through to our detector.

A stage of the mass separator consists of a magnetic dipole, an electric dipole and focusing magnets. At the end of the stage, ions of the same mass but differing initial angles or energies are refocused to the same position, while ions of differing masses arrive at different horizontal positions. Slits are set to allow the desired recoil ions to pass, while blocking the unwanted lower-mass beam ions. The DRAGON separator consists of two stages in series.

There are several detectors found at the end of the mass spectrometer, each measuring a different quantity. They can be used alone or in combinations depending on the precision and type of data needed in the experiment. In addition, there is an array of gamma detectors located around the gas target. These detect the gamma rays given off in most capture reactions. The coincidence between 'seeing' a gamma emission of the right energy and detecting a recoil event gives us confidence that we are detecting recoil atoms, and not beam particles.